Assays capable of detecting the presence of a particular nucleic acid molecule in a sample are of substantial importance in forensics, medicine, epidemiology and public health, and in the prediction and diagnosis of disease. Such assays can be used, for example, to identify the causal agent of an infectious disease, to predict the likelihood that an individual will suffer from a genetic disease, to determine the purity of drinking water or milk, or to identify tissue samples. The desire to increase the utility and applicability of such assays is often frustrated by assay sensitivity. Hence, it would be highly desirable to develop more sensitive detection assays.
Nucleic acid detection assays can be predicated on any characteristic of the nucleic acid molecule, such as its size, sequence, and, if DNA, susceptibility to digestion by restriction endonucleases, etc. The sensitivity of such assays may be increased by altering the manner in which detection is reported or signaled to the observer. Thus, for example, assay sensitivity can be increased through the use of detectably labeled reagents. A wide variety of such labels have been used for this purpose. Kourilsky et al. (U.S. Pat. No. 4,581,333) describe the use of enzyme labels to increase sensitivity in a detection assay. Radioisotopic labels are disclosed by Falkow et al. (U.S. Pat. No. 4,358,535), and by Berninger (U.S. Pat. No. 4,446,237). Fluorescent labels (Albarella et al., EP 144914), chemical labels (Sheldon III et al., U.S. Pat. No. 4,582,789; Albarella et al., U.S. Pat. No. 4,563,417), modified bases (Miyoshi et al., EP 119448), etc. have also been used in an effort to improve the efficiency with which detection can be observed.
Although the use of highly detectable labeled reagents can improve the sensitivity of nucleic acid detection assays, the sensitivity of such assays remains limited by practical problems which are largely related to non-specific reactions which increase the background signal produced in the absence of the nucleic acid the assay is designed to detect. Thus, for some applications, such as for the identification of a pure culture of a bacteria, etc., the concentration of the desired molecule will typically be amenable to detection, whereas, for other potential applications, the anticipated concentration of the desired nucleic acid molecule will be too low to permit its detection by any of the above-described assays.
In response to these impediments, a variety of highly sensitive methods for DNA amplification have been developed.
One method for overcoming the sensitivity limitation of nucleic acid concentration is to selectively amplify the nucleic acid molecule whose detection is desired prior to performing the assay. Recombinant DNA methodologies capable of amplifying purified nucleic acid fragments have long been recognized. Typically, such methodologies involve the introduction of the nucleic acid fragment into a DNA or RNA vector, the clonal amplification of the vector, and the recovery of the amplified nucleic acid fragment. Examples of such methodologies are provided by Cohen et al. (U.S. Pat. No. 4,237,224), Maniatis, T. et al., Molecular Cloning (A Laboratory Manual), Cold Spring Harbor Laboratory, 1982, etc.
Other known nucleic acid amplification procedures include transcription-based amplification systems (Kwoh, D. et al., Proc. Natl. Acad. Sci. (U.S.A.) 86:1173 (1989); Gingeras TR et al., PCT appl. WO 88/10315 (priority: U.S. patent applications Ser. Nos. 064,141 and 202,978)). Schemes based on ligation of two (or more) oligonucleotides in the presence of nucleic acid having the sequence of the resulting "di-oligonucleotide", thereby amplifying the di-oligonucleotide, are also known (Wu, D. Y. et al., Genomics 4:560 (1989)).
Miller, H. I. et al., PCT appl. WO 89/06700 (priority: U.S. patent application Ser. No. 146,462, filed 21 Jan. 1988), disclose a nucleic acid sequence amplification scheme based on the hybridization of a promoter/primer sequence to a target single-stranded DNA ("ssDNA") followed by transcription of many RNA copies of the sequence. This scheme was not cyclic; i.e. new templates were not produced from the resultant RNA transcripts.
Davey, C. et al. (European Patent Application Publication no. 329,822) disclose a nucleic acid amplification process involving cyclically synthesizing single-stranded RNA ("ssRNA"), ssDNA, and double-stranded DNA (dsDNA). The ssRNA is a first template for a first primer oligonucleotide, which is elongated by reverse transcriptase (RNA-dependent DNA polymerase). The RNA is then removed from resulting DNA:RNA duplex by the action of ribonuclease H (RNase H, an RNase specific for RNA in a duplex with either DNA or RNA). The resultant ssDNA is a second template for a second primer, which also includes the sequences of an RNA polymerase promoter (exemplified by T7 RNA polymerase) 5'-to its homology to its template. This primer is then extended by DNA polymerase (exemplified by the large "Klenow" fragment of E. coli DNA polymerase I), resulting as a double-stranded DNA ("dsDNA") molecule, having a sequence identical to that of the original RNA between the primers and having additionally, at one end, a promoter sequence. This promoter sequence can be used by the appropriate RNA polymerase to make many RNA copies of the DNA. These copies can then re-enter the cycle leading to very swift amplification. With proper choice of enzymes, this amplification can be done isothermally without addition of enzymes at each cycle. Because of the cyclical nature of this process, the starting sequence can be chosen to be in the form of either DNA or RNA.
Methods that include a transcription step, e.g. that of Davey, C. et al. (European Patent Application Publication no. 329,822), can increase product by more than a factor of 2 at each cycle. Indeed, as 100 or more transcripts can be made from a single template, factors of increase of 100 or more are theoretically readily attainable. Furthermore, if all steps are performed under identical conditions, no molecule which has finished a particular step need "wait" before proceeding to the next step. Thus amplifications that are based on transcription and that do not require thermo-cycling are potentially much faster than thermo-cycling amplifications which are based on template-dependent primer extension.
In methods which amplify a nucleic acid molecule by template dependent extension, the nucleic acid molecule is used as a template for extension of a nucleic acid primer in a reaction catalyzed by polymerase. For example, Panet and Khorana (J. Biol. Chem. 249:5213-5221 (1974) which reference is incorporated herein by reference) demonstrated the replication of deoxyribopoly-nucleotide templates bound to cellulose. Kleppe et al. (J. Mol. Biol. 56:341-361 (1971) which reference is incorporated herein by reference) disclosed the use of double- and single-stranded DNA molecules as templates for the synthesis of complementary DNA.
The most widely used method of nucleic acid amplification, the "polymerase chain reaction" ("PCR"), involves template dependent extension (Mullis, K. et al., Cold Spring Harbor Symp. Quant. Biol. 51:263-273 (1986); Erlich H. et al., EP 50,424; EP 84,796, EP 258,017, EP 237,362; Mullis, K., EP 201,184; Mullis K. et al., U.S. Pat. No. 4,683,202; Erlich, H., U.S. Pat. No. 4,582,788; and Saiki, R. et al., U.S. Pat. No. 4,683,194), which references are incorporated herein by reference). PCR achieves the amplification of a specific nucleic acid sequence using two oligonucleotide primers complementary to regions of the sequence to be amplified. Extension products incorporating the primers then become templates for subsequent replication steps.
The polymerase chain reaction provides a method for selectively increasing the concentration of a nucleic acid molecule having a particular sequence even when that molecule has not been previously purified and is present only in a single copy in a particular sample. The method can be used to amplify either single or double stranded DNA. The essence of the method involves the use of two oligonucleotides to serve as primers for the template-dependent, polymerase mediated replication of the desired nucleic acid molecule.
The precise nature of the two oligonucleotide primers of the PCR method is critical to the success of the method. As is well known, a molecule of DNA or RNA possesses directionality, which is conferred through the 5'.fwdarw.3' linkage of the sugar-phosphate backbone of the molecule. Two DNA or RNA molecules may be linked together through the formation of a phosphodiester bond between the terminal 5' phosphate group of one molecule and the terminal 3' hydroxyl group of the second molecule. Polymerase dependent amplification of a nucleic acid molecule proceeds by the addition of a 5' nucleoside triphosphate to the 3' hydroxyl end of a nucleic acid molecule. Thus, the action of a polymerase extends the 3' end of a nucleic acid molecule. These inherent properties are exploited in the selection of the two oligonucleotide primers of the PCR. The oligonucleotide sequences of the two primers of the PCR method are selected such that they contain sequences identical to, or complementary to, sequences which flank the sequence of the particular nucleic acid molecule whose amplification is desired. More specifically, the nucleotide sequence of the "first" primer is selected such that it is capable of hybridizing to an oligonuc-leotide sequence located 3' to the sequence of the desired nucleic acid molecule, whereas the nucleotide sequence of the "second" primer is selected such that it contains a nucleotide sequence identical to one present 5' to the sequence of the desired nucleic acid molecule. Both primers possess the 3' hydroxyl groups which are necessary for enzyme mediated nucleic acid synthesis.
In the polymerase chain reaction, the reaction conditions are cycled between those conducive to hybridization and nucleic acid polymerization, and those which result in the denaturation of duplex molecules. In the first step of the reaction, the nucleic acids of the sample are transiently heated, and then cooled, in order to denature any double stranded molecules which may be present. The "first" and "second" primers are then added to the sample at a concentration which greatly exceeds that of the desired nucleic acid molecule. When the sample is incubated under conditions conducive to hybridization and polymerization, the "first" primer will hybridize to the nucleic acid molecule of the sample at a position 3' to the sequence of the desired molecule to be amplified. If the nucleic acid molecule of the sample was initially double stranded, the "second" primer will hybridize to the complementary strand of the nucleic acid molecule at a position 3' to the sequence of the desired molecule which is the complement of the sequence whose amplification is desired. Upon addition of a polymerase, the 3' ends of the "first" and (if the nucleic acid molecule was double stranded) "second" primers will be extended. The extension of the "first" primer will result in the synthesis of a DNA molecule having the exact sequence of the complement of the desired nucleic acid. Extension of the "second" primer will result in the synthesis of a DNA molecule having the exact sequence of the desired nucleic acid.
The PCR reaction is capable of exponential amplification of specific nucleic acid sequences because the extension product of the "first" primer contains a sequence which is complementary to a sequence of the "second" primer, and thus will serve as a template for the production of an extension product of the "second" primer. Similarly, the extension product of the "second" primer, of necessity, contain a sequence which is complementary to a sequence of the "first" primer, and thus will serve as a template for the production of an extension product of the "first" primer. Thus, by permitting cycles of hybridization, polymerization, and denaturation, a geometric increase in the concentration of the desired nucleic acid molecule can be achieved. Reviews of the polymerase chain reaction are provided by Mullis, K. B. (Cold Spring Harbor Symn. Quant. Biol. 51:263-273 (1986)); Saiki, R. K., et al. (BiolTechnology 3:1008-1012 (1985)); and Mullis, K. B., et al. (Met. Enzymol. 155:335-350 (1987), which references are incorporated herein by reference).
PCR technology is useful in that it can achieve the rapid and extensive amplification of a polynucleotide molecule. However, the method requires the preparation of two different primers which hybridize to two oligonucleotide sequences flanking the target sequence. The concentration of the two primers can be rate limiting for the reaction. Although it is not essential that the concentration of the two primers be identical, a disparity between the concentrations of the two primers can greatly reduce the overall yield of the reaction.
All of the above amplification procedures depend on the principle that an end product of a cycle is functionally identical to a starting material. Thus, by repeating cycles, the nucleic acid is amplified exponentially. Methods that use thermo-cycling, e.g. PCR or Wu, D. Y. et al. (Genomics 4:560 (1989)), have a theoretical maximum increase of product of 2-fold per cycle, because in each cycle a single product is made from each template. In practice, the increase is always lower than 2-fold. Further slowing the amplification is the time spent in changing the temperature. Also adding delay is the need to allow enough time in a cycle for all molecules to have finished a step. Molecules that finish a step quickly must "wait" for their slower counterparts to finish before proceeding to the next step in the cycle; to shorten the cycle time would lead to skipping of one cycle by the "slower" molecules, leading to a lower exponent of amplification.
One disadvantage of PCR is that it requires the use of two primers, and thus requires that sequence information be available for two regions of the target molecule. This is often a significant constraint. In some situations, only the amino acid sequence encoded by a target sequence is known. To amplify the target sequence, it is necessary to employ sets of degenerate primers (corresponding to each of the possible sequences capable of encoding the amino acid sequence coded for by the two regions of the target molecule). The use of such degenerate primer sets can cause significant priming errors, and thus an decrease amplification efficiency. One means of decreasing the number of members in the primer sets when conducting PCR amplification is through the use of primers containing deoxyinosine at positions of ambiguity (Patil, R. V., Nucl. Acids Res. 18:3080 (1990); Fordham-Skelton, A. P. et al., Molec. Gen. Genet. 221:134-138 (1990); both of which references are herein incorporated by reference).
A second significant disadvantage of the PCR reaction is that when two different primers are used, the reaction conditions chosen must be selected such that both primers "prime" with similar efficiency. Since the two primers necessarily have different sequences, this requirement can constrain the choice of primers and require considerable experimentation. Furthermore, if one tries to amplify two different sequences simultaneously using PCR (i.e. using two sets of two primers), the reaction conditions must be optimized for four different primers.